Refractory metal articles having oxidation-resistant coating



United States Patent 3,446,606 REFRACTORY METAL ARTICLES HAVING OXIDATION -RESISTANT COATING Leonard A. Friedrich, Hartford, and Emanuel Hirakis, Mansfield Center, Conn., assignors to United Aircraft Corporation, East Hartford, 'Conn., a corporation of Delaware No Drawing. Filed July 14, 1965, Ser. No. 472,405 Int. Cl. C23c 5/00 U.S. Cl. 29-195 14 Claims ABSTRACT OF THE DISCLOSURE Metal articles that exhibit oxidation resistance at high temperatures, and a method of protecting the surface of reactive-refractory metal articles are provided. The articles include a reactive-refractory metal substrate and a coating composition containing a first metal selected from the group consisting of Al, Si, Be, and B, and at least one second metal selected from the group consisting of Sn, Mg, Bi, Li, Ca, Ag, and Cu. The coating also includes 0.5 to 5% of a halide activator, and can include up to by weight in the aggregate of Zn, Ti, and Cr. The first metal forms an intermetallic composition with the substrate when the article is fired. Preferably, the article includes a surface zone comprising an oxidic glass and a subsurface zone comprising the above-described coating composition. A typical article includes a columbium-base alloy substrate, a subsurface zone consisting essentially of 25% by weight Al, 74% by weight Sn, and 1% LiF; and a surface zone of an oxidic glass. In the process, articles are first prepared that conform to the above description, and then fired to form a continuous oxidation-resistant coating.

This invention relates to coatings for the reactiverefractory metals and their alloys that will protect such metals from atmospheric contamination at high temperatures.

More particularly, this invention relates to coatings for the recative-refractory metals that will protect such metals from atmospheric contamination at high temperatures during heat treating and during fabricating, forming and working operations at high temperatures. Such heat treating, fabricating, forming, and working operations at high temperatures are hereinafter referred to collectively as metal forming operations. The ingredients forming the coatings of this invention can be applied to the substrates before heating, and the coatings themselves can then be formed during the heating of the substrates preparatory to metal forming operations. Additional attributes of the coatings of this invention are set forth in the more detailed description which follows.

As used in this specification and claims, the term reacfive-refractory metals refers collectively to the reactive metals titanium and zirconium and the nonprecious refractory metals, or those nonprecious metals having a melting point equal to or higher than the melting point of chromium or 3407 F. (1875 Q). So defined, the refractory metals of this application in ascending order of their melting points are thus: chromium, vanadium, hafnium,

3,446,606 Patented May 27, 1969 columbium, molybdenum, tantalum, and tungsten. The term reactive-refractory metals as used herein refers to alloys having reactive-refractory metal bases as well as to the reactive-refractory metals themselves.

Although silicon and boron a-re borderline elements between metals and nonmetals and are sometimes termed metalloids, for convenience of terminology and precise reference, silicon" and boron are referred to herein as metals. Although silicon and boron, depending upon the functions they perform and the compounds they enter into, are capable of exhibiting both metallic and nonmetallic properties, within the scope of this invention, their properties more closely resemble those of metals than nonmetals. .There is thus justification for referring to these elements as metals in the specification and claims.

For many yea-rs it has been generally known that the high-temperature strength properties of metals are closely related to the melting points. In general, metals having high melting points thus are capable of forming alloys having high strength at high temperatures.

In recent years, the need for new structural materials for service at temperatures in excess of those that can be withstood by conventional structural materials has stimulated interest in those having high melting points, or the reactive-refractory metals and their alloys.

As alloy base materials for high-temperature service, a number of these metals have shown much promise in various high-temperature applications. Perhaps one of the most versatile and promising of these metals is columbium and considerable work has been done to develop it as a structural alloy base for uses in high-temperature environments.

Among the technically more important physical qualities of columbium as an alloy base are its high melting temperature (4474 F.) and its low neutron-capture cross-section. Further, columbium is inherently a soft, ductile, readily fabricable metal and although it becomes too weak for practical structural uses at temperatures much above 1200" F., it is capable of being strengthened for use at much higher temperatures by alloying it with various other metals, particularly by allowing it with other reactive-refractory metals. Disadvantageously, columbium is a highly reactive metal at elevated temperatures and will dissolve realtively large quantities of nitrogen and oxygen on exposure to atmospheres containing even small amounts of these elements at moderately elevated temperatures.

Because of the relative importance of columbium, much of the description that follows is based on the use of the coatings of this invention with columbium-base substrates. It will be understood, however, that the scope of the invention is not limited to coatings for columium-base substrates but includes coatings for the reactive refractory metals generally. The nature of the substrate, particularly as governed by the primary or preponderant element present, determines which forms of the coatings of this invention are most effective for that substrate. For example, although aluminum is a preferred element for forming an intermetallic composition with columbiumbase substates and provides a protective contamination barrier for such substrates at high temperatures, silicon is more preferred as an element for forming a protective tntermetallic composition on molybdenum-base substrates.

It is well known that no metal is completely resistant to surface contamination from exposure to air at elevated temperatures. Most metals that can be used at high temperatures without surface protection form a thin, adherent protective oxide coating during initial exposure. This oxide coating insulates the base metal from further oxidation as long as it remains intact. The pure metals and alloys that exhibit this self-protective attribute are, however, generally limited in their use to temperatures below 1800 F.

At temperatures above 1800 F. the reactive-refractory metals and their alloys are about the only metals that retain sufiicient strength to make theim useful at such temperatures. In recent years the reactive-refractory metals have been subjected to extensive study, investigation and development. Various of the reactive-refractory metals that are in sufficient abundant supply to warrant development have been evaluated for numerous hightemperature uses. Unfortunately, none of the reactiverefractory metals has sufficient resistance to oxidation or contamination in air at high temperatures to be used without protection.

The reactive-refractory metals do not form their own adherent and protective coatings within the temperature ranges of primary interest for their uses. Many of the most promising of these metals, such as columbium and molybdenum, are subject to catastrophic oxidation if unprotected in air at temperatures above 1000 F. And such oxidation vitiates and destroys the high-temperature strength of these metals. Accordingly, many efforts have been directed toward forming effective coatings for the reactive-refractory metals that will inhibit or prevent their oxidation and contamination at high temperatures. Coatings of varying effectiveness have been developed for both shortand long-term protection of reactive-refractory metal substrates.

It is generally agreed that no single coating composition will be found that will provide optimum surface protection to all of the reactive-refractory metals in their various anticipated uses. The coatings must accordingly be tailored to the substrates used and to the immediate application contemplated.

In general, the reactive-refractory metals and thir alloys require heat treating and fabricating operations at elevated temperatures. They are thus typically subjected to rolling, forging, swaging, extruding, and similar fabricating operations at elevated temperatures. characteristically, almost all of these metals at the required temperatures for heat treating and metal forming operations incur atmospheric contamination of serious proportions unless they are protected by an inert or vacuum atmosphere. When metal forming operations are performed on the reactive-refractory metals at high temperatures, gaseous contamination causes formation of a brittle layer on the metal surfaces. This brittle layer renders the metals unsuitable for additional forming operations and almost all potential structural uses.

Accordingly, it has been customary in the prior art to remove this brittle layer after the metal has cooled to render it suitable for further fabrication or use as a structural member. Various methods that have been used for removing the contaminated layer are milling, grinding, sandblasting, and acid etching. Milling results in heavy metal loss since the layer usually must be milled off to its area of greatest depth; sandblasting often results in uneven metal loss; and acid etching can cause pitting and deterioration of the metal surface necessitating grinding to remove these surface defects. These methods of removing the brittle layer not only consume large amounts of expensive metal but create numerous production problems and add further to the cost and time required to produce structural members.

To overcome the disadvantages of conducting metal forming operations on reactive-refractory metals in air or contaminated atmospheres, various alternative procedures have been tried. Among these are encapsulation of the work pieces in protective metal enclosures, such as stainless steel picture frame yokes with cover plates, and fabrication in controlled atmospheres. These methods are typically complex, inconvenient, time-consuming, and expensive.

With some of the reactive-refractory metals, even when a controlled protective atmosphere is used, unacceptable contamination can result. In certain uses of these metals, the presence of even very small amounts of oxygen in the base metal can have serious deleterious results, even though the strength of the metal member remains substantially unimpaired. This is particularly true when a structural member is used for containment of liquid metals. For example, columbium alloys because of their relative strength, availability and fabricability are outstanding candidates as structural materials for liquid metal containment. Other refractory metals have also displayed favorable compatibility with alkali liquid metals, such as lithium.

Pure columbium shows no susceptibility to solution attack by purified lithium at temperatures up to 2200 F. When oxygen in solution in columbium, however, reaches a concentration of as little as a few hundred parts per million, columbium may be rendered sensitive to intergranular lithium attack. Under these conditions lithium will penetrate grain boundaries of columbium base alloys and actually seep through the metal. The attack occurs at all temperatures above 1000 F. and is quite rapid, reaching completion in a few minutes.

This susceptibility to lithium attack in columbium may be reduced by alloying it with zirconium and heat treating. When heat treated, such zirconium containing alloys will fix or tie up interstitial oxygen as zirconium oxide. Zirconium oxide, being more stable than lithium oxide, elfectvely resists lithium attack. Even when alloyed with zirconium, however, columbium will still be susceptible to lithium attack, if oxygen atoms are present in amounts in excess of twice the zirconium atoms present. Accordingly, when columbium is to be used as a structural material for containment of liquid lithium, it is of the utmost importance that the columbium be protected from oxidation at all stages of manufacturing.

When ingots ofthe refractory metals are heated in inert atmosphere furnaces that are not absolutely pure, the ingots will getter any water vapor or any other contaminant in the furnace. At the temperatures and pressures used, the ingots are particularly susceptible to contamination, and even a small amount of oxygen entering, for example, a columbium ingot may render it unsuitable for use with liquid alkali metals.

Moreover, surface oxidation of the refractory metals is so rapid that excessive and unacceptable contamination may occur during transfer of a heated ingot from one inert atmosphere to another during metal forming operations. The problems and expenses in ensuring that a protective atmosphere surrounds an ingot during all steps in a metal forming operation and between steps are obvious.

Accordingly, there has been for many years a need for an effective simplified and economical means for protecting reactive-refractory metals from contamination during heat treating and high-temperature metal forming operations.

The prior art has taught the use of glass coatings on certain of the refractory metals to protect ingots during metal forming operations. Such glass coatings are normally applied in the form of powdered glass or frit. The frit may be sprayed on dry after the substrate is brought to an elevated temperature, or more frequently it is applied in the form of a slurry or a slip before the ingot is heated. Although such glass coatings are alleged to have met with some success, no satisfactory glass coating has been found that offers effective protection to the substrate over the broad range of elevated temperatures within which reactive-refractory metals demand surface protection preparatory to and during metal forming operations. Moreover, most glass coatings disclosed in the prior art are restricted to use with specific metals, because such coatings are incompatible with other reactive-refractory metals and result in contamination of the substrate with elements from the glass coating, notably oxygen.

The problems with the use of glass coatings on reactiverefractory metal substrate can perhaps best be illustrated by a typical example. The behavior of columbium-base substrates with glass coatings will thus be traced as illustrative of the many problems associated with such coatings on reactive-refractory metals.

At temperatures above 500 F. columbium becomes susceptible to contamination by air. When columbium is heated to l6001800 F. oxygen attack becomes so rapid that it may be classified as catastrophic. In the past, it has been normal practice to conduct metal forming opera tions on columbium, such as forging or extrusion, at temperatures of about 2200 F. More recently, however, metal forming operations with columbium-base alloys have been conducted at temperatures as high as 2800 F.

To prepare a columbium-base ingot for metal forming operations, it is thus necessary to heat it through a range of temperatures that may be from room temperature to as high as 2800 F. And at any temperature within this range above 500 F., unprotected columbium becomes susceptible to severe oxidation attack and contamination. When it is remembered that the surface of an ingot reaches temperature well before its core does, and that the larger the ingot, the longer the time period required for its core to reach temperature, it is apparent that contamination may reach serious or unacceptable proportions, if columbium-base substrates are heated for metal forming operations in air, or even a contaminated inert gas, without a protective coating.

In the past, glass frit has been used to create glass coatings on columbium and other reactive-refractory metal base substrates. Results have proved generally unsatisfactory. The problems encountered with glass coatings on columbium illustrate why. These and similar problems are encountered in varying degrees of severity in attempts to protect other reactive-refractory metal substrates with glass coatings.

With columbium alloys the temperature range over which the substrate must be protected may be as extensive as from 500 to 2800 F. Typically, a glass frit that melts or fuses at a temperature low enough to provide protection at 500 F. becomes too fluid or possesses too low a viscosity at the higher end of the temperature range to offer protection. If it fuses at the lower temperatures of the range, it will literally run off the substrate at the higher temperatures.

Glass coatings on columbium and other reactive-refractory metals have also provide unsatisfactory from another important standpoint. Glass frits, particularly those from oxidic glasses, are incompatible with columbium and contaminate it when in direct contact with it at high temperatures.

Moreover, if a defect or localized failure occurs in a glass coating on a reactive-refractory metal substrate, oxidation will dewet the glass in the area of the defect which in turn will cause the defect to expand until a self-feeding reaction sets that may result in virtually complete loss of protection.

Experience has also shown that extensive contamination can occur in columbium-base ingots during heating and metal forming operations in air, even when such ingots are ostensibly protected with glass coatings. This contamination often penetrates the substrate to depths that require large amounts of the ingots to be milled off to render them free of contaminated surface layers. Frequently it is necessary to mill off as much as 100 mils of surface.

It has also been found that glass coatings on columbium-base ingots cannot even adequately protect such ingots from contamination in inert or controlled atmospheres. Such atmospheres seldom approach absolute purity, and when heated to high temperatures, ingots tend to getter any Water vapor or other contaminants in the furnace. During heating, columbium-base ingots are particularly susceptible to contamination, and any contaminant, such as the oxygen in water vapor, tends to penetrate deeply into the ingot. When such a contaminated ingot is formed into a containment structure for liquid metals, even a slight inclusion of oxygen, as previously set forth, can result in liquid metal leaching and seepage, which can quickly cause localized failure of the containment structure.

Finally, even when inert atmospheres can be made sufliciently pure and free from contaminants to allow substantially contamination-free heat treating and metal forming operations on reactive-refractory metals, such procedures have generally proved impractical. In production operations it is highly expensive and difficult to pump a vacuum furnace down to a low enough pressure to assure that it will have the requisite degree of freedom from contaminants.

In view of the foregoing, it is a primary object of this invention to provide new and improved protective coatings for reactive-refractory metal substrates at heat treating and metal forming temperatures whereby such substrates can be transferred from furnaces to metal forming equipment and worked in air Without danger of oxidation or contamination.

It is a further object of this invention to provide new and improved coatings for reactive-refractory metal substrates and a process for applying such coatings. These coatings provide excellent protection against oxidation and contamination of such substrates during heat treating and metal forming operations at high temperatures. In a preferred form, the coatings afford protection to the substrates over the full temperature range within which the substrates are susceptible to oxidation and contamination from air or contaminated atmospheres.

It is another object of this invention to provide new and improved protective coatings for reactive-refractory metal substrates that combine advantages of intermetallic composition coatings with advantages of liquid metal modified intermetallic composition coatings and in a preferred form also with advantages of glass coatings and avoid the disadvantages of each of these coatings.

Another object of this invention is to provide a coating for reactive-refractory metal ingots that comprises in a preferred form an oxidic glass surface zone and as a subsurface zone, an intermetallic composition modified by a liquid metal forming composition, the coating serving to protect the surface of the ingot over a wide range of elevated temperatures from temperatures at which oxidation begins up to the highest temperatures used for metal forming operations.

A further object of this invention is to provide a protective coating for the reactive-refractory metals and their alloys that will prevent or drastically reduce oxidation and contamination during elevated temperature metal forming operations and which may be readily removed from the metal surface after cooling upon completion of such metal forming operations.

Another object of this invention is to provide a coating for reactive-refractory metal substrates that permits heat treating and high-temperature metal forming operations to be carried out without the need for expensive and time-consuming post-heating treatments to remove contaminated surface layers by procedures such as milling, grinding, sandblasting, caustic or acid cleaning, and the like.

A further and important object of the invention is to provide new and improved coatings for reactive-refractory metal substrates that will protect them from oxidation during heating in air furnaces preparatory to hightemperature metal forming operations and during transport of such substrates from furnaces to metal forming or metal fabricating equipment.

Another object of this invention is to provide novel coatings for the reactive and refractory metal substrates that will protect them and permit use of conventional equipment normally used for heating and fabricating medium-temperature alloys, such as cobalt and nickel base alloys (the so-called superalloys which do not normally require protection from oxidation) without the need for surrounding such equipment with an inert protective atmosphere.

Yet another object of this invention is to provide novel coatings for reactive-refractory metal substrates that will protect such substrates from atmospheric contamination while high-temperature metal forming operations are actually being carried out on the substrate itself.

A still further object of a preferred form of this invention is to provide improved coatings for reactive-refractory metal substrates that will protect them from contamination during heat treating and high-temperature metal forming operations with a COating having a selfhealing metallic subsurface zone backed-up by a selfhealing glass surface zone to prevent contamination of the substrate by cracking of coatings and formation of defects during metal forming operations.

Other subjects of a preferred form of this invention are:

(1) To provide a novel coating for reactive-refractory metal substrates that includes a glass outer surface which exhibits sufiiciently low viscosity at temperatures for metal forming operations to ensure coverage of the substrate while it is being subjected to plastic deformation;

(2) To provide a coating that has a glass surface zone having a sufficiently low fusing temperature to protect a subsurface zone from oxidation while the temperature of the elements forming the coating is raised to a level at which at least one of the metals in such subsurface zone forms an intermetallic composition with the substrate;

(3) To provide a coating having a glass surface zone that acts as a lubricant during high-temperature extrusion and similar metal forming operations and prevents the coating from being torn away from the substrate during such operations; and

(4) To provide a coating having a glass surface zone that forms a vitreous outer layer that can be readily removed when the substrate is cooled to room temperature to leave a clean surface on the substrate.

Additional objects and advantages will be set forth in part in the description that follows, and in part will be obvious from the description, or may be learned by practice of the invention, the objects and advantages being realized and attained by means of the compositions, methods and processes particularly pointed out in the appended claims.

To achieve the foregoing objects and in accordance with its purpose, in a preferred embodiment this invention includes, as broadly described, a coated metal article having a substrate selected from the group consisting of the reactive-refractory metals and alloys thereof and a coating having an exterior layer or surface zone comprising:

(1) At least one first metal selected from the group consisting of Al, Si, Be, and B;

(2) At least one second metal selected from the group consisting of Sn, Mg, Bi, Li, Ca, Ag, and Cu; and preferably (3) At least one halide selected from the group consisting of the alkali metal halides and the alkaline earth metal halides.

In accordance with the invention, the aggregate of first metals in the surface zone is from 20 to 60% by weight of the composition of the surface zone. And for supe- 8 rior results it is preferred that such first metals be present in an aggregate amount of 25 to 40% by weight of the surface zone.

Similarly, the aggregate of second metals in the surface zone is from 35 to 80% by weight of the coating composition of the surface zone. And for superior results it is preferred that such second metals be present in an aggregate amount of from 5 8 to by weight of the surface zone.

Also, the aggregate of halides in the surface zone is from 0.5 to 5% by weight of composition of the surface zone. And for superior results it is preferred that such halides be present in an aggregate amount of from 0.5 to 2% by weight of the surface zone.

In accordance with the invention, the coatings may optionally include as metallic additions to the surface zone one or more elements selected from the group consisting of Zn, Ti, and Cr. Such optional additional elements should not exceed 10% by weight in the aggregate of the composition of the surface zone, and preferably do not exceed 7% by weight.

It will be understood that the selected second metal of the coating should be one which has a boiling point substantially above the working temperature to be used with the particular substrate selected.

In a preferred form of the invention the coating also includes an oxidic glass outer layer or surface zone. When such an oxidic glass surface zone is used the surface zone earlier referred to then becomes a subsurface zone.

The glass outer coating makes it possible to heat the substrate in conventional furnaces without inert atmosphere. If the substrate is heated in an inert atmosphere furance, however, it will not be necessary to use a glass outercoat to protect it until an intermetallic coating forms. Nevertheless, the most beneficial results of the invention are obtained when the glass outercoat is included, since the glass provides back-up protection and lubrication during metal forming operations.

This invention further embraces a process for protecting the surface of a reactive-refractory metal article from gaseous contamination during heat treating and high-temperature metal forming operations. In a preferred form this process comprises the steps of:

(1) Applying to the substrate a first coating composition comprising:

(a) finely divided particles of at least one first metal selected from the group consisting of Al, Si, Be, and B;

(b) finely divided particles of at least one second metal selected from the group consisting of Sn, Mg, Li, Ca, Ag, and Cu; and

(c) at least one halide selected from the group consisting of the alkali metal halides and the alkaline earth metal halides;

(2) applying over a first coating composition a second coating composition comprising an oxidic glass frit; and

3) firing the substrate, with the first and second coating compositions in place, to a predetermined temperature, whereby a coating is produced on the substrate which has a metallic subsurface zone and a substantially glass surface zone and which is substantially impervious to gaseous contaminants, such as oxygen, at elevated temperatures.

In accordance with the invention, the first and second coating composition will also preferably include a vaporizable diluent, in an amount sufficient to give the composition a spreadable consistency, and a binding or sticking agent. The binding or sticking agent causes particles of the first coating composition to adhere both to each other and to the substrate and causes particles of the second coating composition to adhere both to each other and to the first coating composition.

For the preferred form of the invention, many different types of oxidic glasses are suitable for creating the surface zone of the coatings. Typical glasses from which frit may be formed are the so-called soda-lime glasses and borosilicate glasses. In general, almost any oxidic glass that has a softening point of from about 1000 to 1500 F. is suitable for forming the frit used to create the surface zone of the coatings of this invention.

In accordance with the invention, the glass frit should have the property of beginning to fuse or coalesce at a temperature lower than the softening point of its glass base or at a temperature of from about 600 to 1400 F. characteristically, most oxidic glasses display such a tendency to agglomerate, cohere, or fuse at temperatures lower than their softening point.

Frit of soda-lime glasses falling within the following ranges of composition are preferred for the coatings of this invention:

SODA-LIME GLASSES Composition: Percent by weight sio 6876 Na O 10-18 CaO 8-13 MgO 2-4 A1 0.5-3

BOROSILICATE GLASSES Composition: Percent by weight sio 65-82 B203 Na O d 26 A1 0 1-4 K 0 0-4 In accordance with the invention, excess SiO' A1 0 or other high-melting refractory oxides can be mixed with frit formed from lower melting glasses. The glass frit in such mixtures of frit and refractory oxides on the outer surface of an ingot melts at a relatively low temperature and thus provides interim protection for the subsurface zone of the coating while the temperature increases to one at which the intermetallic composition forms as a subsurface zone.

When the glass frit reaches a higher temperature approaching the melting temperature of the supplemental refractory oxides, it will dissolve these refractories. Such dissolved supplemental refractory oxides then change the composition of the original frit to a higher melting or more viscous glass. In the meantime, an intermetallic composition providing a primary protection barrier is formed in the subsurface zone. The more viscous glass resulting from the combination of frit and oxide provides better high-temperature protection than is offered by the glass of the original frit.

In accordance with the invention, the first metal of the subsurface zone forms an intermetallic composition with the substrate that is oxidation resistant and provides a contamination barrier. Usually, the intermetallic composition is the last part of the coating formed during heating of an ingot that has been prepared by contacting it with the coating ingredients.

Such intermetallic composition provides primary protection in the coatings of the invention, but since it does not form until substantially high temperatures are reached, the substrate must be given interim protection over the lower part of the temperature range at which oxidation takes place, when an air furnace is used. Otherwise, if the substrate becomes oxidized before reaching the formation temperature of the intermetallic composition, oxygen contamination at the substrate surface may prevent formation of the desired intermetallic.

In accordance with the invention, the second metal of the subsurface zone (or surface zone if glass is not used) is selected to melt well below the formation temperature of the intermetallic composed of the first metal and the substrate. When the second metal melts, preferably near the lower end of the oxidation temperature range, it forms an oxidation barrier on the surface of the substrate.

When in liquid phase, it is believed the second metal forms a liquid metal diffusion path by which finely divided particles of the first metal are carried into intimate contact with the substrate and with each other thereby enhancing the first metal-substrate reaction that forms the inermetallic. It is also believed that the second metal forms a solution or a eutectic with the first metal that beneficially lowers the melting point of the first metal, thereby further serving to enhance rapid and efiicient formation of the intermetallic composition.

The liquefied second metal promotes and supports diffusion of first metal particles to the substrate, partly by convection movement of the first metal particles within the liquid of the second metal.

Also in accordance with the invention, the third component of the coating subsurface zone (or surface zone if glass is not used) is an activating agent comprising an alkali metal halide or alkaline earth metal halide. The halide activating agent serves to flux the metal powders, particularly those of the first metal, and the substrate and promotes coalescence, wetting, and fusion and the reaction of the metal powders with the substrate to create the desired intermetallic composition.

The activating agent or halide also serves to reduce oxide films on the metal powder particles, particularly those of the first metal, as the ingot is heated up, thereby promoting the desired intermetallic reaction.

In accordance with the invention, metals that may be optionally added to the coating, namely, Zn, Ti, and Cr, achieve various functions that improve the coating, such as increasing long-term oxidation resistance, promoting self-healing properties, and deoxidizing.

To aid in understanding the invention, a preferred embodiment of the coatings will be described as exemplary of the invention.

On a columbium-base alloy substrate a subsurface zone consisting essentially of the following composition by weight yields desired results:

Percent Al 25 Sn 74 LiF 1 As illustrative of What is needed for higher tempera ture metal forming operations, such as at temperatures up to 2800 F., a borosilicate glass frit of the following composition by weight has been found to yield superior results:

Percent SiO B 0 l4 N320 4 A1 0 2 lit problem with unprotected columbium-base substrates. Next, the glass frit fuses or agglomerates and knits together at about 1000 F. and provides a surface barrier that keeps the aluminum powder from oxidizing. Oxidation of aluminum powder can prevent coalescence of the individual grains into molten aluminum, even after melting temperature is exceeded.

In accordance with the invention, the glass frit forming the outer layer of the sprayed-on ingot coating heats up more quickly than the interior parts of the coating and ingot and provides interim protection for the substrate, together with melted tin, before formation temperature for the Al-Cb intermetallic composition is reached. The MP acts as an activator or flux and promotes the reaction between aluminum powder and the surface of the columbium-base ingot causing the intermetallic compound CbAl to form at a temperature low enough to provide eifective primary protection.

Attempts to use fluoride glasses as activators or fluxes have generally proved unsuccessful, because the fluoride glasses react with and corrode the intermetallic compound, decreasing its effectiveness.

The final coating achieved on columbiurn-base substrates with the composition just described is a doublelayered coating having essentially the following composition from the outer surface to the substrate: Borosilicate glass, Sn-t-Al solution, CbAl Cb Sn.

In accordance with the invention, the glass outer surface provides back-up protection for defects that may occur in the intermetallic composition and subsurface zone, since the glass acts as a viscous fluid at temperatures used for metal forming operations. Also in accordance with the invention, the outer glass coating, in a fluid or essentially liquid form at higher temperatures, provides lubrication for metal ingot surfaces during fabrication. This lubricating function of the glass coating is particularly important in high-temperature extrusion operations.

In the composite coatings of the invention, the glass overcoat thus performs the following functions:

(1) It protects the dried slurry or slip of mixed metal powders of the first coating composition while these powders inter-react and react with the substrate to form an intermetallic composition that provides primary hightemperature protection.

(2) It acts as a lubricant during metal forming operations, particularly during extrusion at high temperatures.

(3) It serves as a back-up to the second metal-first metal liquid phase, such as for example, Sn-Al, and provides an outer coating that acts as a fluid at high temperatures which covers and heals defects in the coating induced during metal forming operations or otherwise.

The embodiment of the coating composition just described for :columbium-base alloys has proven effective in protecting .columbium-base ingots at temperatures at least up to the melting point of CbA13 which is about 2900 F.

In accordance with the invention, the coatings of the invention are particularly adapted to protect substrates throughout the various steps used in high-temperature metal forming operations and not just during heat treating or transport of an ingot from one metal forming operation to another.

For example, when high-temperature extrusion is used in a metal forming operation, cracks or gaps are likely to occur in the subsurface zone or intermetallic portion of the coating. With the coatings of the invention, such breaches in the primary protection barrier are covered by operation of self-healing properties of the liquid metal phase and glass surface coating.

As earlier described, and in accordance with the invention, it is believed the liquid metal phase may comprise:

(l) The second metal alone,

(2) The second metal in solution with the first metal,

(3) A eutectic composition of the second and first metals, or

(4) A more complex liquid phase, such as, a eutectic and an excess of a first or second metal.

The outer glass coating typically exhibits reasonably low viscosity at extruding temperatures, giving it fiuid properties that promote rapid healing of extrusion-induced defects.

In accordance with the invention, when a coated ingot is forged, the intermetallic portion of the coating may crack, because at most forging temperatures it possesses little ductility. The versatility of the coatings of the invention is illustrated, however, by the capability of the subsurface-zone liquid metal phase and the outer glass coating to provide fluid coverage of defects created in the intermetallic portion of the coating.

Further in accordance with the invention, there is believed to be a continuous reaction between coating and substrate which begins when the melting temperature of the second metal, or a combination of second and first metals, is reached and continues as the temperature rises. With the Sn-Al embodiment of the coating on columbium, the liquid phase maintained in the subsurface zone is believed to consist mainly of Sn with Al in solution. This liquid phase provides the requisite fluidity to promote self-healing of defects, particularly those defects induced during working or forming operations on the ingots, either with or without a glass overcoating.

Accordingly, even when the intermetallic portion of the coating becomes cracked, as for example, during a forging operation, additional intermetallic continues to form in the defects while the coating remains at temperature. Additional CbAl intermetallic is created by liquid phase feeding of Alto the substrate either in solution with Sn, or by convection of Al powder in liquid Sn, whereby a continuous reaction between A1 and the columbiumbase substrate is promoted.

Upon completion of metal forming operations and cooling of the ingot, the coatings of this invention may be removed by various methods, if desired. In most instances when a soda-lime glass is used, the hardened glass coating may be shattered and spalled oif. This may be done either mechanically or intrinsically by thermal shock brought on by rapid cooling, which can be induced by compressed air or a quenching operation. Not infrequently, it may be desirable to remove only the glass outer portion of the coating and to retain the intermetallic portion substantially intact. A second coating may then be applied on top of the retained intermetallic portion of the coating in preparation for further mechanical forming operations.

The metal powders of the first and second metals used in the coatings of this invention preferably have a size range that will permit them to pass through a 200 mesh screen, although coarser particles up to a size that will pass through a mesh screen may also be used. Es pecially good results are obtained when the size range of the metal powders is reduced to a size that will pass through a 325 mesh screen (43 microns), or between about from 0 to 43 microns, and preferably between about 0 to 10 microns. In general, it can be said that the finer the particles, the better will be the final coating produced. The mesh sizes reported are Tyler standard.

The metal and halide activator dusts or powders described above may be applied to reactive-refractory metal substrates to be treated in any suitable manner. A fine film of the powders may thus be blasted or dusted on to the substrate, or a dispersion of the powders in a diluent or solvent liquid may be applied to the substrate, after which the diluent or solvent may be evaporated leaving a coating of the powder mixture on the substrate.

The amount of diluent or solvent used with the powders can be adjusted to suit the particular application, such as brush application, spraying, dipping, or other appropriate means, for spreading the coating on the surface of the substrate. Other methods of applying the powder mixture to the substrate will readily suggest themselves to persons skilled in the art.

Before coating, the surfaces of the substrate should be thoroughly cleaned of dust, dirt, or other foreign substances as by water rinsing, liquid blasting, washing in suitable organic and inorganic solvents, and immersion in alkali cleaners or acid pickles. Care should be taken in cleaning the substrate to ensure removal of all foreign matter.

In accordance with a preferred embodiment of this invention, an Al and Sn powder mixture is dispersed in a suitable liquid diluent, and the resulting dispersion is applied to the substrate by spraying, brushing, dip-coating, or other effective method.

The diluent used in the preparation of a powder mixture dispersion can be any compatible diluent including water. Any of the well-known diluents employed with resins and polymers in the paint industry may be used. Preferably, a readily volatilizable organic solvent or mixture of solvent is used. Among non-limiting examples of solvents that may be used are lower aliphatic alcohols, lower aliphatic ketones, lower alkyl esters of lower aliphatic acids, and lower hydrocarbons such as benzene and lower alkyl substituted benzenes. Non-limiting examples of such diluents are methhyl, ethyl, propyl, and butyl alcohol; acetone, methyl ethyl ketone, diethyl ketone, octyl, hexyl ketone; methyl acetate, butyl acetate, octyl acetate; methyl propionate, octyl hexanoate, benzene, tolulene, xylene, ethyl benzene, and the like.

The organic solvents mentioned are illustrative and not limiting. The main requirement of the volatile liquid substance or diluent is that it be reasonably safe to use, inexpensive, and sufficiently liquid at ordinary temperatures to act as a dispersant for the metallic powders so that the dispersant can be sprayed or suitably coated on the specimen, and at the same time be sufficiently volatile to evaporate when exposed to atmospheric or other conditions as will be described below.

In a preferred embodiment of this invention, the halide activator is LiF. LiF is soluble in many of the organic solvents mentioned above, and preferably when LiF is used an organic solvent will be selected in which LiF is readily dissolved. Similarly, when other halides of the alkai metals and alkaline earth metals are used as activators, preferably such halides should be soluble in the organic solvent to be used as a diluent.

If desired, a binding or sticking agent may be added to the liquid diluent to hold the powder mixture to the surface of the substrate after evaporation of the solvent. Use of a binder enables the powders to adhere to the substrate for prolonged periods of time, thereby preeluding the necessity of heat treating immediately after application of powders or of taking special precautions in handling the treated substrate to avoid loss of powders.

The binder should be one that will substantially completely decompose during heat treating and that will preferably decompose at a temperature below the melting point of the lowest melting metal or combination of metals used. Suitable binding or sticking agents that may be mentioned include nitrocellulose, naphthalene, and stearates. Other sticking or binding agents will be readily apparent to those skilled in the art.

Suitable wetting agents may also be added to the diluent if required. Moreover, low boiling organic compounds in small amounts can be added to the diluent to enhance its rapid evaporation.

If the sticking agent is added to the diluent, upon evaporation of the solvent, the sticking agent will remain dispersed throughout the powder mixture in the coating, and will serve to hold the powder or dust on the substrate before heat treating begins.

Evaporation of the volatile solvent, or a volatile portion of the lacquer, containing a sticking agent, may be conveniently brought about by allowing the coated substrate to be stored in an atmospheric environment at room temperature. If desired, suction or vacuum and elevated temperatures, may be used to accelerate evaporation of the volatile solvent. Evaporation of the solvent leaves a fine layer of metallic powder mixture, such as for example Sn and Al, on the surface of the substrate or ingot to be treated.

When a hinder or sticking agent is added to the liquid diluent, the layer of powders, upon evaporation of the solvent, comprises a substantially uniform mixture of coating powders interspersed throughout the nonvolatile hinder or sticking agent. The dry coating adhering to the substrate comprise metallic and halide particles and binder, with such particles being suspended or interspersed throughout the binder.

The ratio of metallic and halide powders to liquid diluent may vary from about 1:1 to 1:10, or higher, with the amount of diluent being adjusted to suit the particular method of application. A ratio of powder to diluent of 1:1 is satisfactory when it is desired to use a spatula to spread the coating on the surface to be protected. For spray application, however, the coating composition will be of proper consistency when the ratio of powders to diluent is about 1:10. Still larger amounts of diluent may be used if desired, however, amounts of diluent in excess of a powder to diluent ratio of 1:10 are of no particular advantage and increase the amount of diluent that must be evaporated from the coating.

The metallic and halide powders may be mixed in the diluent or lacquer by any of the arts well known in the paint industry, or simply by using a Waring Blendor.

The thickness of applied powders comprising the first coating composition may vary from substrate to substrate, but in general, a sprayed or applied thickness of from 2 to 5 mils is contemplated. Such sprayedor brushed-on coating compositions lead to a final thickness after heat treating of from about 1 to 3 mils. Preferably the thickness of the resulting subzone of the coatings of this invention after heat treating is about 2 mils.

A preferred lacquer or diluent with binding or sticking agent for use with the coating compositions of this invention is nitrocellulose lacquer or nitrocellulose dissolved in an organic solvent such as amyl acetate.

When evaporation of the solvent has been completed and when a glass overcoat is desired, the resulting specimens are ready for application of the second coating composition or glass frit mixed with diluent. The earlier description of various suitable diluents and methods of application for the first coating composition apply also to preparation and application of the second coating composition. In other words, diluents suitable for use with the first coating composition are also suitable for use with the second coating composition, and the methods of application suitable for use with the first coating composition are also suitable for use with the second coating composition.

For especially good results, the glass frit of the second coating composition is reduced in a ball mill to a particle size that will pass through a screen of about mesh or smaller. Preferably the smaller the better, and for best results the glass frit should be about 200 mesh, although frit of a size from about -50 to 200 mesh or smaller is acceptable.

As previously set forth, soda-lime or borosilicate glasses having softening points sufficiently low that their frits display the capacity to agglomerate, fuse, or coalesce at a temperature of from about 700 to 1500 F., preferably 700 to 1100 F., are suitable. In accordance with this invention, to create suitable glass frits for the outer layer of the coating which will exhibit sufficient viscosity to provide protection at higher elevated temperatures during metal forming operations on the higher-melting of the reactive-refractory metals, the viscosity of the glass at a given temperature may be controlled to some extent by increasing its content of SiO and B The melting temperature and viscosity of the glass can also be raised sharply by adding A1 0 in very small amounts above 3% by weight.

After the second coating composition comprising glass frit and a suitable diluent or lacquer has been applied to the substrate on top of the dried first coating composition, the solvent portion of the second coating composition is evaporated. The now dried second coating composition of glass frit and binder or sticking agent comprises the outer surface layer of the substrate or specimens.

The resulting specimen is ready for heat treatment in a suitable furnace or oven to prepare it for metal forming operations. As previously described, and in accordance with the invention, the specimen can be heated in an air furnace and raised to its heat treating temperature in such a furnace. The previously described coating embodiment for columbium-base substrate begins to develop at about 400-500 F. and continues to develop with increasing temperature until the CbAl intermeta-llic is formed between A1, as the first metal, and columbium of the substrate.

The thickness of the applied frit of the second coating composition may vary from substrate to substrate, but in general, a brushed-on thickness of from 10 to 15 mils is contemplated. Such sprayedor brushed-on coating composition produces a glass surface zone having a thickness after heat treating of from about 6 to 10 mils. Preferably, the thickness of the resulting glass surface layer of this invention after heat treating is about 7 mils.

For a clearer understanding of the invention, specific examples of it are set forth below. These examples are merely illustrative and are not to be understood as limiting the scope and underlying principles of the invention in any way.

EXAMPLE 1 A columbium alloy ingot measuring approximately 3" in diameter and 6" long and consisting essentially of 1% zirconium by weight and balance essentially all columbium was prepared for a 2200 F. forging in accordance with the steps described below.

The ingot was degreased and its surface grit blasted lightly with a No. 90 alumina grit.

A mixture of high purity metallic powders and a halide of the following composition was prepared:

76% by weight of tin powder (-200 mesh or finer),

15% by weight of aluminum powder (pigment grade aluminum paste of 325 mesh or finer),

% by weight of zinc powder (-200 mesh or finer),

2% by weight chromium powder (-100 mesh or finer) and 2% by weight of LiF powder. a

This metal powder mixture was suspended in a nitrocellulose lacquer (nitrocellulose dissolved in amyl acetate) by mixing in a Waring Blendor. To provide a quantity of first coating composition suitable for spraying, approximately 100 grams of dry powder per 40 cc. of lacquer were mixed together. After mixing, the coating was applied to the ingot by spraying at the rate of 10 to milligrams of coating composition per square centimeter of substrate surface. The coating composition thus applied had a sprayed-on thickness of about 4 mils. This first coating composition was then permitted to dry in air for one hour at about 70 F.

At the end of this time substantially all of the organic solvent had evaporated from the nitrocellulose lacquer leaving a coating composition on the ingot of metal powders, LiF and nitrocellulose as a binder sticking agent. The LiF powder in the coating composition dissolved in the nitrocellulose lacquer when the powder was mixed with the lacquer to form the coating composition. When the solvent evaporated the LiF precipitated out and remained substantially evenly distributed throughout the coating composition.

After the solvent had evaporated from the first coating composition, the ingot was sprayed with a second coating composition consisting essentially of a powdered sodalime glass frit suspended in nitrocellulose lacquer in a ratio of about grams of frit per 50 cc. of lacquer. The composition of the soda-lime glass used was as follows:

Composition: Percent by weight SiO 72 Na O l5 CaO 9 MgO 3 A l- 0 1 After mixing in a Waring Blendor this second coating composition was sprayed on the ingot at a ratio of approximately 65 to 75 milligrams of coating composition per square centimeter of substrate surface. The second coating composition thus applied had a sprayed-on thickness of about 10 mils. The second coating composition was then permitted to air dry at about 75 F. for two hours.

The ingot with its adhering coats of powder was placed in a gas fired rotary furnace at about 2200 F. The furnace was already at temperature, in accordance with preferred practice, when the ingot was put in.

After 52 minutes exposure in the air furnace, the ingot was removed and upset forged to a 3" height on an 18,000 ton Mesta Press. Subsequent metallographic examinations of the ingot revealed that contamination had occurred to a maximum depth of 200 mils. Surface cracks were observed to a maximum depth of 13 mils. The oxygen content in the 50 to 80 mil layer of the ingot was found not to exceed ppm. These results showed that the coating furnished good protection from oxidation contamination to the ingot during the forging operation.

EXAMPLE 2 A tantalum alloy ingot consisting essentially of 8% by weight of tungsten, 2% by weight of hafnium, and balance essentially all tantalum, was prepared for forging at 2400 F. in accordance with the procedures set forth in Example 1. A first and second coating composition of the same ingredients as in Example 1 was applied to the tantalum-base alloy ingot in the same manner set .forth in Example 1. Upon heat treating, a coating having the desired properties was formed on the surface of the ingot.

EXAMPLE 3 A molybdenum ingot consisting essentially of 0.5% by weight of titanium, balance essentially all molybdenum, was prepared for forging at 2400 F. in accordance with the procedures set forth in Example 1.

As a first coating composition, the following mixture was prepared:

60% by weight of tin powder (-200 mesh or finer),

38% by weight of silicon powder (-200 mesh or finer), and

2% by weight of LiF powder.

This metal powder mixture was suspended in a nitrocellulose-amyl acetate lacquer by mixing in a Waring Blendor. The first coating composition was then applied to the ingot in the manner described in Example 1.

A soda-lime glass frit, of the type described in Example 1, was suspended in the same lacquer base to form the second coating composition and was applied over the dry powders of the first coating composition.

After drying of the second coating composition, the ingot was fired in the manner described in Example 1 and forged with results similar to those obtained in Example 1.

17 In addition to the foregoing examples, the following embodiments of the invention also satisfy its objects and yield its-new and useful result:

EXAMPLE 4 A columbium alloy ingot consisting essentially of 1% by weight of zirconium and balance essentially all columbium to which-is applied a first coating composition consisting essentially of Al as the first metal, Cu as the second metal, and NaF as the halide activator.

EXAMPLE 5 An ingot composed predominantly of tungsten to which is applied a first coating composition consisting essentially of Si as the first metal, Ag as the second metal, and KP as the halide activator.

EXAMPLE 6 A columbium alloy ingot consisting essentially of 5% by weight of zirconium and balance essentially all columbium to which is applied a first coating composition consisting essentially of B as the first metal, Bi as the second metal, and CaF as the halide activator.

EXAMPLE 7 An ingot composed predominantly of Ti to which is applied a first coating composition consisting essentially of Al as the first metal, Bi as the second metal, and LiF as the halide activator.

EXAMPLE 8 A metal article as described in Example 4 to which is also applied a second coating composition consisting essentially of a soda-lime glass frit.

EXAMPLE 9 A metal article as described in Example 5 to which is also applied a second coating composition consisting essentially of a borosilicate glass frit.

EXAMPLE 10 A metal articles as described in Example 6 to which is also applied a second coating composition consisting essentially of a borosilicate glass frit.

EXAMPLE 11 A metal article as described in Example 7 to which is also applied a second coating composition consisting essentially of a soda-lime glass frit.

EXAMPLE 12 An ingot composed predominently of hafnium to which is applied a first coating composition consisting essentially of Be as the first metal and Cu as the second metal and a second coating composition consisting essentially of a borosilicate glass frit.

EXAMPLE 13 An ingot composed predominantly of vanadium to which is applied a first coating composition consisting essentially of B as the first metal and Ca as the second metal and a second coating composition consisting essentially of a soda-lime glass frit.

EXAMPLE 14 18 What is claimed is: 1. A metal article having a substrate selected from the group consisting of columbium, molybdenum, tantalum, titanium, chromium, vanadium, zirconium, hafnium, and tungsten and alloys having at least one of the metals of said group as their base, the substrate having a coating layer applied thereon consisting essentially of:

(a) from 20 to 60% by weight of at least one first metal selected from the group consisting of Al, Si, Be and B,

(b) from 35 to by weight of at least one second metal selected from the group consisting of Sn, Mg, Bi, Li, Ca, Ag, and Cu, and

(c) from 0.5 to 5% by weight of at least one halide activator selected from the group consisting of the alkali metal halides and the alkaline earth metal halides.

2. A metal article having a substrate selected from the group consisting of columbium, molybdenum, tantalum, titanium, chromium, vanadium, zirconium, hafnium, and tungsten and alloys having at least one of the metals of said group as their base, the substrate having a high-temperature protective coating including:

(a) a surface zone comprising an oxidic glass; and

(b) a subsurface zone consisting essentially of:

(i) from 20 to 60% by weight of at least one first metal selected from the group consisting of Al, Si, Be, and B,

(ii) from 35 to 80% by weight of at least one second metal selected from the group consisting of Sn, Mg, Bi, Li, Ca, Ag, and Cu, and

(iii) from 0.5 to 5% by weight of at least one halide activator selected from the group consisting of the alkali metal halides and alkaline earth metal halides.

3. A coated metal article which comprises a substrate selected from the group consisting of columbium, molybdenum, tantalum, titanium, chromium, vanadium, zirconium, hafnium, and tungsten and alloys having at least one of the metals of said group as their base, the article having a coating layer applied thereon consisting essentially of:

(a) from 20 to 60% by weight of at least one first metal selected from the group consisting of Al, Si, Be, and B, the first metal forming an intermetallic composition with the substrate, (b) from 35 to 80% by weight of at least one second metal selected from the group consisting of Sn, Mg, Bi, Li, Ca, Ag, and Cu, and

(c) from 0 to 10% by weight in the aggregate of at least one third metal selected from the group consisting of Zr, Ti, and Cr.

4. A coated metal article which comprises a substrate selected from the group consisting of columbium, molybdenum, tantalum, titanium, chromium, vanadium, zirconiurn, hafnium, and tungsten and alloys having at least one of the metals of said group as their base, the article having:

(a) an oxidation and defect-failure resistant surface zone comprising an oxidic glass, and

(b) a subsurface zone consisting essentially of:

(i) from 20 to 60% by weight of at least one first metal selected from the group consisting of Al, Si, Be, and B, the first metal forming an intermetallic composition with the substrate,

(ii) from 35 to 80% by weight of at least one second metal selected from the group consisting of Sn, Mg, Bi, Li, Ca, Ag, and Cu, and

(iii) from 0 to 10% by weight in the aggregate of at least one third metal selected from the group consisting of Zn, Ti, and Cr.

5. A metal article having a substrate selected from the group consisting of columbium, molybdenum, tantalum, titanium, chromium, vanadium, zirconium, hafnium, and tungsten and alloys having at least one of the metals of said group as their base, and having a high-temperature protective coating zone consisting essentially of:

(a) from 25 to 40% by weight of at least one first metal selected from the group consisting of Al, Si, Be, and B,

(b) from 58 to 75% by weight of at least one second metal selected from the group consisting of Sn, Mg, Bi, Li, Ca, Ag, and Cu, and

(c) from 0.5 to 2% by weight of at least one halide selected from the group consisting of the alkali metal halides and the alkaline earth metal halides.

6. A metal :article having a substrate selected .from the group consisting of columbium, molybdenum, tantalum, titanium, chromium, vanadium, zirconium, hafnium, and tungsten and alloys having at least one of the metals of said group as their base, the substrate having a hightemperature protective coating comprising:

(a) a surface zone comprising an oxidic glass;

(b) a subsurface zone consisting essentially of:

-(i) from 25 to 40% by weight of at least one first metal selected from the group consisting of Al, Si, Be, and B,

(ii) from 58 to 75% by weight of at least one second metal selected from the group consisting of Sn, Mg, Bi, Li, Ca, Ag, and Cu, and

(iii) from 0.5 to 2% by weight of at least one halide selected from the group consisting of the alkali metal halides and alkaline earth metal halides.

7. A metal article having a substrate selected from the group consisting of columbium, molybdenum, tantalum, titanium, chromium, vanadium, zirconium, hafnium, and tungsten and alloys having at least one of the metals of said group as their base, the substrate having a hightemperature protective coating zone consisting essentially of:

(a) from 15 to 60% by weight in the aggregate of at least one first metal selected from the group consisting of Al, Si, Be, and B,

(b) from 35 to 80% by Weight in the aggregate of at least one second metal selected from the group consist-ing of Sn, Mg, Bi, Li, Ca, Ag, and Cu,

(c) from to 10% by weight in the aggregate of at least one metal selected from the group consisting of Zn, Ti, and Cr, and

(d) from 0.5 to 5% by weight in the aggregate of at least one halide activator selected from the group group consisting of the alkali metal halides and the alkaline earth metal halides.

8. A metal article having a substrate selected from the group consisting of columbium, molybdenum, tantalum, titanium, chromium, vanadium, zirconium, hafnium, and tungsten and alloys having at least one of the metals of said group as their base, the substrate having a hightemperature protective coating including:

(a) a sunface zone comprising an oxidic glass; and

(b) a subsurface zone consist-ing essentially of:

(i) from 20 to 60% by weight of at least one first metal selected from the group consisting of Al, Si, Be, and B,

(ii) from 35 to 80% by weight of at least one second metal selected from the group consisting of Sn, Mg, Bi, Li, Ca, Ag, and Cu,

(iii) from 0 to by weight in the aggregate of at least one third metal selected from the group consisting of Zn, Ti, and Cr, and

(iv) from 0.5 to 5% 'by weight of at least one halide activator selected from the group consisting of the alkali metal halides and alkaline earth metal halides.

9. A metal article having a substrate selected from the group consisting of col-umbium and columbium-base alloys, and having a high-temperature protective coating zone consisting essentially of:

(a) from 20 to 60% by weight of Al,

(b) from 40 to by weight of Sn, and

(c) from 0.5 to 5% by weight of at least one halide selected from the group consisting of the alkali metal halides and the alkaline earth metal halides.

10. A metal article having a substrate selected from the group consisting of columbium and columbium-base alloys, the substrate having a high-temperature protective coating including:

(a) a surface zone comprising at least one glass composition selected from the group consisting of the soda lime glasses and borosilicate glasses; and

4 (b) a subsunface zone consisting essentially of:

(i) from 20 to 60% by weight of Al, (ii) from 40 to 80% 'by weight of Sn, and (iii) from 0.5 to 5% by weight of at least one halide selected from the group consisting of the alkali metal halides and alkaline earth metal halides.

11. A metal article having a substrate selected from the group consisting of tantalum and tantalum-base alloys, the substrate having a high-temperature protective coating comprising:

(a) a surface zone consisting essentially of a glass composition selected from the group consisting of the soda-lime glasses and the borosilicate glasses; and

(b) a subsurface zone consisting essentially of:

(i) from 20 to 40% by weight of Al,

(ii) from 40 to 80% by weight of Sn, and

(iii) from 0.5 to 5% of at least one halide selected from the group consisting of the alkali metal halides and alkaline earth metal halides.

12. A metal article having a substrate selected from the group consisting of molybdenum and molybdenum-base alloys, the substrate having a high-temperature protective coating comprising:

(a) a surface zone consisting essentially of at least one glass composition selected from the group consisting of the soda-lime glasses and the borosilicate glasses; and

(b) a subsurface zone consisting essentially of:

(i) from 20 to 40% by weight of Si,

:(ii) [from 40 to 80% by weight of Sn, and

(iii) :from 0.5 to 5% by weight of at least one halide selected from the group consisting of the alkali metal halides and alkaline earth metal halides.

13. The article of claim 3 wherein the second metal includes Sn.

14. The article of claim 3 wherein the second metal includes at least one of Mg, Bi, and Li.

References Cited UNITED STATES PATENTS 3,294,497 12/ 1966 Foldes 29-194 3,344,505 10/1967 Riveley 29-195 X 3,096,324 2/1963 Morgan '117--71 X 3,006,782 10/ 1961 Wheildon 29-195 X 2,696,662 1'2/ 1954 Le Sech.

HYLAND B'IZO I, Primary Examiner.

US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,446,606 May 27 1969 Leonard A. Friedrich et al.

It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 19, line 46, cancel rou ":-""co1umri 2o, "rimsemand 55, the claim reference numeral 3", each occurrence, slhould read l Signed and sealed this 14th day of April 1970.

(SEAL) Attest:

WILLIAM E. SCHUYLER, JR.

Edwin-d M. Fletcher, Jr.

Commissioner of Patents Attesting Officer 

